A conventional double heterostructure light emitting diode is described on pages 1217 to 1222 of "Journal of Lightwave Technology, Vol. LT-3 No.6" published in December of 1985. The double heterostructure light emitting diode comprises a buffer layer of Sn doped n-InP, an active layer of Zn doped p.sup.+ -InGaAsP, a cladding layer of Zn doped p-InP, a confining layer of Sn doped n-InGaAsP, a clapping layer of Zn doped p-InGaAsP, and a SiO.sub.2 film respectively in turn provided on an n-InP substrate. The double heterostructure light emitting diode further comprises a p-electrode provided on the SiO.sub.2 film, an n-electrode having a light emitting window provided on the back surface of the n-InP substrate, and a Zn diffused region of a predetermined diameter built in the confining layer wherein the p-electrode is partly projected into a current injecting aperture of the SiO.sub.2 film to be in contact with the Zn-diffused region extending through the capping layer over the confining layer.
According to the double heterostructure light emitting diode described above, the active layer is heavily doped with a p-impurity like Zn to shorten the lift time of carriers to that a high speed response is obtained. As a consequence, the rise and fall times of the light output may be as fast as 350 ps using a specified speedup circuit and applying DC bias offset.
In the conventional double heterostructure light emitting diode, however, the light output is considerably decreased because the active layer is heavily doped with a p-impurity such as Zn etc., although the response time is increased with the increase of the impurity concentration therin. Further, the crystal surface of the active layer tends to be rough due to the high concentration doping so that fabrication yield is decreased. For this reason, there a limitation in obtaining a high speed response in a light emitting diode comprising an active layer to be doped with such a p-impurity as Zn etc.
Alternatively, if an active layer is not heavily doped with a p-impurity a peaking of pulses must be performed in a rise and fall thereof in a circuit for driving the light emitting diode, or a DC bias voltage must be applied thereto to obtain a high speed response.